1. Field of the Invention
This invention relates to a reproduced chroma signal processing automatic phase controlling circuit to be used for a circuit reproducing an original carrier chroma signal from a low range converted chroma signal reproduced from a magnetic tape in a VTR (Video Tape Recorder) or the like.
2. Description of the Related Art
In a domestic VTR, a chroma signal is converted in the frequency to be in a low range and is recorded on a magnetic tape. In the reproduction, on the contrary, the low range chroma signal is converted in the frequency to the original carrier chroma signal and is reproduced. Here, an NTSC system television signal shall be explained.
In the recording, a carrier chroma signal is separated and extracted from a compound video signal and is multiplied by a local signal of a predetermined frequency to obtain a carrier chroma signal converted in the frequency to be in a low range. This low range converted chroma signal is recorded as mixed with a luminance signal separated and extracted from the compound video signal and modulated in the frequency. In the reproduction, the reproduced low range chroma signal is multiplied by the output of a local oscillator of a predetermined frequency controlled by an automatic phase controlling (abbreviated as APC hereinafter) circuit to obtain a chroma signal having the original carrier frequency. An APC circuit compares the phase of a chroma burst signal of a chroma signal, returned to its original frequency, to a reference signal. The APC circuit also controls a local oscillator, used for altering the frequency of the low range chroma signal, in accordance with the output of the comparison to maintain frequency and phase of the output chroma signal in agreement with the frequency and phase of the reference signal to remove a time-axis fluctuating component of the reproduced chroma signal.
If guard-bandless recording is performed with no guard band (space area) between adjacent tracks in order to achieve high density recording, crosstalk from the adjacent tracks is generated during reproduction of the chroma signal. An azimuth recording system is normally used to perform such guard-bandless recording. In this azimuth recording system, however, because an azimuth signal which is converted to a low range frequency has low azimuth loss, crosstalk from the adjacent tracks can be generated and appears on an image plane as noise. Therefore, in the recording, the chroma signal is recorded with the phase displaced so that the crosstalk component of the reproduced chroma signal may be inverted in the phase in each horizontal scanning period and the crosstalk component is removed by passing the chroma signal through a comb filter at the time of the reproduction. As an example of this recording system, there is a chroma phase shifting system (PS system) in the VHS (Video Home System). In this PS system, the low range chroma subcarrier frequency is selected to be 1/2 integral times ((1/2).times.80 times) horizontal frequency(=629 kHz) and the phase of the chroma signal is recorded as shifted by 90 degrees in each horizontal period (1H). At the time of the reproduction, the phase of the chroma signal is recovered in 1H, the subcarrier frequency is returned to the origin, the present carrier chroma signal and the 1H delayed and inverted carrier chroma signal are superimposed on each other by using a comb filter using a 1H delay line and the crosstalk component is canceled to obtain only the chroma signal component from the reproducing track.
Such chroma signal processing APC circuit and chroma crosstalk removing recording and reproducing system as in the above are shown, for example, in U.S. Pat. No. 4,178,606.
FIG. 6 shows an APC loop of a color reproducing circuit used for a conventional VHS system VTR. FIGS. 7 and 8 show respectively formation examples of a comb filter 4 and APC detector 7 in FIG. 6. In FIG. 6, a reproduced low range converted chroma signal (of 629 KHZ) from a magnetic tape is input into an input terminal 1 and is fed to a frequency converter 2 in which the input low range chroma signal is multiplied by a local signal (of 4.2 MHZ) obtained by dividing the frequency of an oscillated signal from a voltage controlled oscillator (abbreviated as VCO hereinafter) 9 and having the phase displaced by 90 degrees per H so as to be the original carrier chroma signal of 3.58 MHZ. This carrier chroma signal has only the component of 3.58 MHZ extracted through a band pass filter (BPF) 3, further has the crosstalk component canceled through a comb filter 4 and is led out to an output terminal 5.
In the comb filter 4, as shown in FIG. 7, a carrier chroma signal (output of BPF 3) of 3.58 MHZ including a crosstalk is input into an input terminal 41, is delayed by 1H by a 1H delaying line 42, is then inverted by an inverting circuit 43 and is fed to one input end of an adder 44. A carrier chroma signal before the 1H delay from the input terminal 41 is input as it is into the other input end of the adder 44, which combines the above mentioned 1H delayed and inverted carrier chroma signal and outputs to an output terminal 45 (that is, the above mentioned output terminal 5) as a carrier chroma signal of 3.58 MHZ having the crosstalk removed.
On the other hand, the carrier chroma signal output from the comb filter 4 is fed to an APC detector 7.
In the APC detector 7, as shown in FIG. 8, a carrier chroma signal (output of the comb filter 4) of 3.58 MHZ is input into an input terminal 71 and a burst signal is extracted from the carrier chroma signal by a burst gate circuit 72. The burst signal extracted by the burst gate circuit 72 is compared in the phase with a reference signal (output of a crystal oscillator 6) from an input terminal 73 in a phase detector 74 and is output as a phase detecting output from an output terminal 75.
In the APC detector 7, the burst part of the output of the comb filter 4 is compared in the phase with the reference signal of the crystal oscillator 6. The phase comparing output is smoothed by the filter 8 and is then fed to an oscillation controlling terminal of the VCO 9. The output of the VCO 9 is divided in the frequency by the frequency divider 10, then the phase is shifted by 90 degrees per H by the phase shifter 11 and is fed to the above mentioned frequency converter 2 as a local signal (of 4.2 MHZ).
In such formation, if a phase fluctuation occurs in the carrier chroma signal of 3.58 MHZ output from the comb filter 4, the phase difference from the reference signal will be output as a phase detecting output from the APC detector 7. The phase detecting output from the APC detector 7 delays by the phase difference the phase of the oscillated frequency of the VCO 9 so that the carrier chroma signal output from the comb filter 4 may be controlled to be of the frequency and phase of the reference signal.
Here, when the phase of the low range converted chroma signal reproduced from a magnetic tape is represented by .phi.c and the phase of the local signal fed back so as to be multiplied to it is represented by .phi.o, on the phase of this APC loop, the closed loop gain G(s)=.phi.o/.phi.c shall be determined. When the phase detection sensitivity(=output voltage variation/phase difference variation) of the phase detector within the APC detector 7 is represented by .mu., the control sensitivity(=frequency variation/controlling voltage variation) of the circuit part composed of the VCO 9 and the frequency divider 10 is represented by .beta. and the characteristic of the filter 8 is represented by F(s), it will be given by ##EQU1## wherein s=j .omega.(=j2.pi. f) and M(s) represents the characteristic of the comb filter 4. When the period of 1H is represented by TH, it will be ##EQU2## H(s) represents a sample holding effect produced for the APC detection of the burst part of each H and is ##EQU3## If the characteristic F(s) of the filter 8 is flat until the high range, it will be simplified as F(s)=1 and, if the formulae (2) and (3) are substituted in the formula (1), ##EQU4##
In a sufficiently low frequency, as EQU exp(-2TH.multidot.s).apprxeq.1-2TH.multidot.s,
this is similar to ##EQU5## becomes a negative imaginary number value and has a phase of about -90 degrees. When the frequency becomes high, this phase will further delay. When s=j.multidot.(.pi./2).multidot.fH (wherein fH=1/TH and fH=15.75 kHZ), that is, when the frequency f=(1/4).multidot.fH, if s=j.multidot..pi./(2TH) is substituted in the formula (4), G(s) will become a negative real number of ##EQU6## and the phase will become -180 degrees (that is, the same phase) and will become a positive feedback. Therefore, here the gain must be set to be smaller than 1. Reversely speaking, the response of the APC loop can not be made more than (1/4).multidot.fH. In fact, in order to keep the APC loop system stable, only a lower frequency response can be made.
This frequency response can be extended to a higher range with the comb filter 4 deleted but can not be adopted unless, for example, the crosstalk part is substantially nil and therefore is not general.
As mentioned above, the response of the APC loop of the color reproducing circuit in the VTR, can not be made until a high frequency and therefore there has been a problem that a color irregularity remains on the picture.